1. Field of the Invention
The invention is generally in the field of hydraulic cement and concrete.
2. Relevant Technology
Supplementary Cementitious Materials (“SCMs”), such as fly ash, slag, natural pozzolans, and limestone, are often used to replace a portion of Portland cement in concrete. SCMs can yield improved concrete with higher durability, lower chloride permeability, reduced creep, increased resistance to chemical attack, lower cost, and reduced environmental impact. Pozzolans react with calcium hydroxide released during cement hydration. Limestone can provide a filler effect and nucleation sites.
Portland cement, sometimes referred to as “cement clinker” or “OPC” (acronym for “ordinary Portland cement”), is the most expensive component of concrete. The manufacture of cement clinker contributes an estimated 5-7% of all manmade CO2. There is a long-felt but unmet need to reduce cement clinker (“clinker”) consumption. There have been numerous academic conferences and publications dedicated to the concept of substituting a portion of clinker with SCM. Despite an oversupply of low cost SCMs, the industry has failed to overcome technical hurdles to more effectively utilize SCMs. This failure, after years of research and discussion, to fully utilize readily available and less expensive SCMs to reduce clinker consumption, even though doing so would reduce cost and benefit the environment, means that conventional practices for utilizing SCMs are inadequate. Hundreds of millions of tons of waste SCMs such as fly ash and steel slag continue to be discarded into the environment worldwide each year at a cost to the producer and even greater cost to the environment.
SCMs are typically waste products not purposely produced for blending with OPC. Because OPC and SCMs are often produced for different reasons by different industries, OPC manufacturers have little or no influence on SCM production and SCM producers have little or no influence on OPC manufacture. The result is that cement manufacturers continue to produce and optimize OPC for use with itself without regard to how OPC behaves when substituted with SCMs.
Cement manufacturers deliberately produce OPC having a broad particle size distribution (“PSD”) (e.g., between about 1-60 μm) in an attempt to strike a balance between the competing effects and demands of reactivity, rate of strength development, water demand, inter particle spacing, paste density, porosity, autogenous shrinkage, and grinding cost. PSD and chemistry are selected to optimize use of OPC by itself. SCM substitution is secondary and has little or no influence on how cement is manufactured. Slightly raising Blaine fineness when intergrinding clinker and SCM to offset retardation is the only commercial attempt to “optimize” cement for SCM substitution.
SCMs are usually less reactive than clinker and retard strength development by dilution. Although some OPC-SMC blends can approach the strength of OPC at later ages (>56 days), early (1-28 day) strength can be severely impacted when more than about 10-20% of OPC is replaced with SCM. Early strength loss and/or delayed set times limit SCM usage in concrete. The conventional solution is to “fix” the SCMs to make them more reactive, e.g., by grinding them more finely, either independently or by intergrinding with clinker. Neither solution has solved problems of SCM underutilization. Meanwhile, waste SCMs continue to accumulate worldwide in alarming quantities, and the disconnect between OPC production and effective SCM utilization persists.